The nearly exponential growth in the heat generated by miniaturized electronic devices in recent years demands significant improvements in cooling technology. Existing fan-assisted air cooling methods will be insufficient for the next generation of microprocessors. Only liquid-cooled heat exchangers will be able to absorb and dissipate heat rapidly enough to maintain safe microprocessor operating temperatures. A stringent requirement of high efficiency is imposed on such a heat exchanger. The cooling system must be small and must be a closed loop, so that it may: a) fit within a desktop or laptop computer; and b) not require external cooling water.
Microchannel Heat Exchangers
Decreasing the liquid cooling channel dimensions to the micron scale in a solid-liquid heat exchanger leads to high heat transfer rates. Convective heat transfer from the channel surface to water is fast, but diffusional heat transfer from the liquid at the interface to liquid in bulk is slow. By reducing the liquid cooling channel dimensions, the interface area-to-bulk volume ratio increases, thereby reducing rate-limiting diffusional heat transport.
There have been prior demonstrations of high solid-fluid heat transfer from microchannels, primarily in silicon-based microchannel heat exchangers. The use of silicon in such devices in these studies was not so much because silicon has desirable heat transfer properties, but rather because fabrication techniques for Si-based, high-aspect-ratio microscale structures (HARMS) are relatively mature and widely available. Indeed, Si possesses a substantially lower bulk thermal conductivity than that of the metals that would otherwise be preferred in larger-scale heat exchangers, such as Cu and Al. Further, Si is relatively brittle, and consequently Si-based devices tend to be fragile and easily damaged.
Si microfabrication techniques typically involve a photolithography process in which a uniform, polymerizable resist layer is deposited onto a Si substrate, and a desired pattern is photoexposed into the resist layer. Unpolymerized resist is removed chemically or by solvation, and the Si substrate is etched through the developed resist pattern either by wet chemical etching (WCE) or reactive ion etching (RIE). Additional deposition and etching of thin metal films may be required for the RIE process. Photolithography and etching are required for each Si microscale device, and to enjoy an economy of scale, a substantial investment in large clean room and thin film deposition facilities is required.
Microchannel heat exchangers have also been fabricated in materials other than Si by the LiGA process. LiGA combines deep X-ray/UV lithography (Lithographie) of a polymeric resist, followed by metal electrodeposition (Galvanoformung) into the developed resist recesses to form durable, primary HARMS. Replication of secondary HARMS from the primary HARMS via molding (Abformung) then follows. For example, U.S. Pat. No. 6,415,860 discloses Ni electrodeposition to make microscale Ni mold inserts that are then used to mold microchannel heat exchangers in polymethylmethacrylate (PMMA). Metal-based crossflow heat exchangers, such as those made from NiP alloys, were also made, by an additional electroless deposition onto LiGA-fabricated polymer templates. F. Arias et al., “Fabrication of metallic heat exchangers using sacrificial polymer mandrils,” JMEMS vol. 10, p. 107 (2001) reported the fabrication of Ni-based heat exchangers by electrodeposition of nickel onto sacrificial polymer mandrels.
There are unfilled needs in existing heat exchangers. For example, the thermal conductivity of PMMA is poor, and PMMA-based microchannel heat exchangers cannot endure temperatures higher than about 100° C. While Ni-based and NiP-based heat exchangers can function at higher temperatures, their heat conductivities are still less than optimal. Furthermore, the electrode-based and electroless deposition techniques used to make them are slow, and require close monitoring and control. Their cost of fabrication is high and is expected to remain high because of the extra deposition steps involved in these “lost-mold” processes.
Existing Si microfabrication techniques do not work for making metal-based microstructures. For example, the structural and chemical isotropy of polycrystalline metals leads to removing material in a somewhat isotropic manner in a WCE process, broadening features from those defined lithographically. RIE techniques are also inappropriate for metallic substrates. Because metal-based microchannel devices are highly desirable for heat transfer applications, there is an unfilled need for improved fabrication techniques to mass-produce metal-based microchannel devices rapidly and inexpensively.
Microchannel Fabrication by Compression Molding
Microscale compression molding, or hot embossing, of polymeric plastic materials is an established technique. First, a primary HARMS mold insert is produced, typically through a sequence of lithography, etching, deposition steps, with optional additional steps. Second, the mold insert is impressed into a substrate, and polymer fills voids in the mold insert through viscous or plastic flow to form the negative of the insert pattern. A large number of negative HARMS replicas can be reproduced from a single primary HARMS. In principle, under favorable conditions one primary mold insert may be used to produce hundreds or even thousands of replicas rapidly and at low cost.
The quality of the replica depends upon, among other factors, the mechanical yield strength of the mold insert at elevated molding temperatures. An important problem confronting compression metal microstructure molding is the lack of microstructure mold insert materials that retain high mechanical yield strengths at the molding temperatures required for metals. An electrodeposited Ni mold insert, for example, suffers permanent shape deformation when used to mold a higher-melting temperature metal, such as Cu.
Another problem can arise from chemical reactivity between the mold insert and the metal substrate. During compression, chemical bonds can form between the insert and the substrate. These bonds can cause the insert to break and can damage the molded structure as it is withdrawn from the substrate. These surface chemistry problems had restricted the metals that could be used as mold inserts and as substrates, until the development of a conformal ceramic surface coating to inhibit chemical bond formation. Using ceramic conformal coatings, secondary HARMS have been successfully reproduced in previously problematic, chemically reactive metals, such as Zn and Al, with LiGA-fabricated Ni mold inserts. See generally D. Cao et al., “Amorphous hydrocarbon based thin films for high-aspect-ratio MEMS applications,” Thin Solid Films 398-399 (2001) 553-559; and D. Cao et al., “Conformal deposition of Ti—C:H coatings over high-aspect-ratio microscale structures and tribological characteristics,” Thin Solid Films 429 (2003) 46-54.
Bonding the Cover Plate
Once a microchannel has been fabricated in a substrate, whether Si or metal, a leak-tight cover plate must be affixed before it can be used as a practical heat exchanger. Several bonding methods have been reported for Si-based microsystems, including anodic bonding and direct bonding. However, these techniques are not well-suited for bonding metal-based HARMS.
Eutectic Bonding
Braze-bonding of bulk metal pieces has previously been used in different applications. Brazing is a joining process in which a non-ferrous filler metal or alloy is heated to its melting temperature and distributed between two (or more) close-fitting metal parts by capillary action. The filler metal can optionally be a eutectic mixture. A “eutectic” mixture is a mixture whose proportions are such that the melting point is as low as possible; and such that the constituents of the mixture all crystallize simultaneously at this temperature from molten liquid solution, a temperature that is called the eutectic point. For example, it has been reported that thin films of Si, Si—Al, and Zn—Al have been deposited onto bulk Al pieces by electron beam evaporation or sputtering. These Al pieces were then braze-bonded to one another by heating to 578-595° C., with flux introduced to remove surface aluminum oxides. This technique would be unsuitable for use with microchannels, however, because flux residue would tend to block the microchannels.
D. Tuckerman et al., “High performance heat sinking for VLSI,” IEEE Elect. Dev. Lett. 2, 126-129 (1981) discloses a water-cooled, integral heat sink fabricated in silicon with a Pyrex cover plate.
A. Tiensuu et al., “Assembling three-dimensional microstructures using gold-silicon eutectic bonding,” Sensor Actuat A 45, 227-236 (1994) discloses the use of gold-silicon eutectic bonding to join silicon microelements to one another.
B. Vu et al., “Patterned eutectic bonding with Al/Ge thin films for microelectromechanical systems,” J Vac Sci Technol B 14(4):2588-2594 (1996) discloses the use of an aluminum/germanium eutectic to bond silicon dice to one another.
P. Lee et al., “Investigation of heat transfer in rectangular microchannels,” Int. J. Heat Mass Transf., vol. 48, no. 9, pp. 1688-1704 (2005) discloses measurements and numerical modeling of heat transfer in rectangular microchannels. Test pieces were made of copper, with ten microchannels in parallel, and a polymeric cover plate.
D. Cao et al., “Microscale compression molding of Al with surface engineered LiGA inserts,” Microsyst Technol. 10 (2004) 662-670 discloses the use of high-temperature compression molding of aluminum plates with high-aspect ratio microscale mold inserts made of nickel conformally coated with a titanium-containing hydrocarbon. See also W. Meng et al., “Stresses during micromolding of metals at elevated temperatures: pilot experiments and a simple model,” J. Mater. Res. 20 (2005) 161-175; J. Jiang et al., “Further experiments and modeling for microscale compression molding of metals at elevated temperatures,” J. Mater. Res. 22 (2007) 1839-1848; U.S. Pat. No. 7,114,361; and U.S. published patent application 2005/0056074.
F. Mei et al., “Eutectic bonding of Al-based high aspect ratio microscale structures,” Microsyst Technol. 13: 723-730 (published online 16 Jan. 2007) reports work from our research group concerning the eutectic bonding of Al-based high aspect ratio microscale structures with Al—Ge intermediate layers. See also F. Mei et al., “Evaluation of eutectic bond strength and assembly of Al-based microfluidic structures, Microsyst Technol. 14: 99-107 (published online 3 Apr. 2007); F. Mei et al., “Fabrication, assembly, and testing of Cu- and Al-based microchannel heat exchangers, J. Microelectromechanical Systems 17(4): 869-881 (published online Jun. 27, 2008); and F. Mei et al., “Evaluation of bond quality and heat transfer of Cu-based microchannel heat exchange devices,” J Vac Sci Technol A 26(4):798-804 (published online Jun. 30, 2008).
U.S. Patent Application 2006/0142401 discloses the use of partial boiling in a minichannel or microchannel to remove heat from an exothermic process. Surface roughness was said to enhance nucleation for boiling. See, e.g., Example 11.
U.S. Patent Application 2006/0157234 discloses a microchannel heat exchanger, and briefly mentions surface roughness.
U.S. Pat. No. 5,727,618 discloses a modular microchannel heat exchanger formed from a stack of multiple thin copper sheets etched with rows of elongated holes, coated with silver and held together with the holes aligned, e.g. with pins. The stack is heated, and the copper and silver form a fused or eutectic alloy brazing the sheets together. The holes through the multiple sheets then form a microchannel.